Echo suppressor circuits



July 20, 1937. B. G. BJORNSON 2,087,314

ECHO SUPPRESSOR CIRCUITS Filed June '20, 1956 INVENTOR B. G. BJORNSONATTORNEY Patented July 20, l937 UNITED STATES PATENT OFFICE ECHOSUPPRESSOR CIRCUITS Application June 20, 1936, Serial No. 86,234

9 Claims.

The invention relates to two-way signaling systems and particularly tothe circuits employed for directionally controlling signal transmissionin such systems.

An object of the invention is to improve the operation of a long two-waysignal transmission system employing therewith signal-operated apparatusfor suppressing echoes.

To obtain proper operation of a long two-way 1Q telephone systemincluding over at least a portion of its length, a four-wire circuitcomprising oppositely directed one-way paths for the signals in oppositedirections, it has been found necessary in the past to employ therewithvoice-operated 15 switching apparatus to insure that only one of thesepaths is operative at a time. This is to prevent echoes of thetransmitted voice signals from being carried back to the transmittingend of the system and causing a disturbance. The voice-operatedswitching apparatus when located at the terminals of the four-wirecircuit are known as terminal echo suppressors.

The invention is particularly concerned with terminal echo suppressorsof the differential type. These echo suppressors usually comprise ateach terminal, a polar relay having one winding adapted to be energizedin response to wave energy in the outgoing one-way path by a controlcircuit connected thereto, and an opposing 30 winding adapted to beenergized in response to Wave energy in the incoming path of theterminal by a control circuit connected to that path, the relay tendingto operate in one direction or the other depending on which windingcarries the 35 stronger current. In one system of this type in the priorart, the outgoing path at each terminal is normally operative and ismaintained operative by the relay in response to stronger signals inthat path, but is disabled by operation of the relay 40 when the signalsin the incoming path are stronger.

The invention is specifically directed to improvements in suchdifierential type terminal echo suppressors, the improvements consistingin the 45 provision of means for facilitating break-ins by a listeningparty after a talking party has obtained control of the switchingcircuit, and without decreasing the efficiency of the circuit as regardsthe suppression of echoes. This is ac- 50 complished mainly by areceiving control circuit operating on alternate syllables of incomingvoice signals to apply a bias against break-ins to the transmittingcontrol circuit proportional to'the amplitude of the stronger ofsuccessive syllables, 55 which is applied quickly enough to preventfalse operation thereof by the initial echo of the incoming signals, andis prolonged just sufilciently to prevent false operation by theterminal echo.

A more complete understanding of the invention with its various objectsand features will 5 be had from the detailed description thereof whenread in connection with the accompanying drawing, the single figure ofwhich shows schematically one terminal portion of a four-wire tolltelephone system equipped with a terminal echo suppressor in accordancewith a preferred form of the invention.

The four-wire toll telephone circuit of the drawing comprises a one-Waysignal transmission path EA for repeating telephonic currents in thedirection from west to east and a one-way transmission path WA forrepeating telephonic currents in the direction from east to west. Theinput of the path EA and the output of the path WA are coupled at thewest terminal of the four Wire circuit in conjugate relation with eachother and in energy transmitting relation with the two- Way telephoneline TL by the hybrid coil H1 and associated balancing network N1 inwell-known manner. The transmission path EA includes near the westterminal the one-way amplifying device I and the loss network 2, and thetransmission path WA includes near the west terminal the one-wayamplifying device 3.

Connected across the path EA in front of the amplifying device I throughthe directional network 4 is the input of the transmitting controlcircuit 5 of the terminal echo suppressor. The directional network l maybe of any suitable type which will allow transmission from the incomingportion of the path EA into the control circuit 5 while preventingsubstantially transmission from the outgoing portion of the path EA intothe control circuit 5. The particular network illustrated is a specialbridge transformer such as is disclosed and claimed in Crisson Patent1,755,743 issued April 22, 1930.

The transmitting control circuit 5 comprises a main portion includingthe three-electrode vacuum tube amplifier 6, a three-electrode vacuumtube detector 1, the input of which is coupled to the output of theamplifier 6 by transformer 8, and the polarized suppressor relay SR, onewinding 9 of which is connected in the plate-cathode circuit of thedetector tube 1. Connected across the plate circuit of the vacuum tubeamplifier 6 is an auxiliary circuit l0 including the threeelectrodevacuum tube detector I l and a polarized relay I2 having its windingconnected in series with the plate-cathode circuit of the detector tubel Connected in the grid-cathode circuit of the detector tube l in serieswith the secondary winding of the transformer 8 is the copper-oxiderectifying device i3, and connected across the gridcathode circuit oftube 11 in shunt with the secondary winding of transformer 8 and therectifying device l3 in series, is the control condenser I4. Thecondenser M is normally shunted by the resistance 15 through thenormally closed contact and armature of the polarized relay I2.

Connected across the path WA in the output of the amplifying device 3through the directional network It, similar to the network i, is theinput of the receiving control circuit ll of the terminal echosuppressor. The receiving control circuit i"! includes the threeelectrode vacuum tube amplifier l8 and the two operating channels l9 and20, the inputs of which are coupled in conjugate relation with eachother and in energy transmitting relation with the output of theamplifier 28 by the hybrid coil network H2 and associated balancingnetwork N2. The operating channels l9 and 20 are so connected to enableeither channel to be blocked without affecting the input of the other.

The receiving control circuit ll also includes a selector circuit 2|,the function of which is to direct alternate incoming speech syllablesreceived from the path WA into the proper receiving channel. Theselector circuit 2 l, the input of which is connected across theplate-cathode circuit of the amplifier it in front of the hybrid coil H2includes a three-electrode vacuum tube detector 22, the selector masterrelays SM1 and 8M2, the windings of which are connected in series acrossthe plate-cathode circuit of the detector tube 22, one of the latterwindings being always shorted, which control the operating channels i8and 2B in the manner which will be described later, and adifferentially-connected selector polarized relay SP which alternatelyconditions the relays SlVIi and SMz for operation.

The operating channel 28 includes a series copper-oxide rectifyingdevice 23 for charging the condenser 24 connected in shunt with thechannel, and a three-electrode vacuum tube detector 25 having its inputelectrodes connected across the condenser M, and the output circuit ofwhich is connected or adapted to be connected under control of theamplitude comparor polarized relay ACP, across the other winding 25 ofthe suppressor relay SR. The winding 25 of the suppressor relay SR iswound in opposing relation to the winding 9 thereof controlled by thetransmitting control circuit 5.

The receiving channel IS includes a series copper-oxide rectifier 21 forcharging the condenser 28 connected in shunt with the channel, and athree-electrode vacuum tube detector 28 having its input circuitconnected across the condenser 28 and its output circuit connected oradapted to be connected by the relay ACP across the winding 26 of thesuppressor relay SR as will be explained.

The receiving channel 26 is normally shortcircuited in the input of therectifier 23 through the normally closed armature and contact of thequick-operating channel control polarized relay RQOi, and is normallyshort-circuited in the output of the rectifier 23 through the normallyclosed armature and contact of the polarized slow-release channelcontrol relay RSR1 and its series resistance R1. Similarly, theoperating channel I!) is normally short-circuited in. the input ofrectifier 2'! through the normally closed armature and contact of thequick-operating polarized channel control relay RQOZ, and is normallyshort-circuited in the output of the rectifier 2l through the normallyclosed armature and contact of the slow-release polarized channelcontrol relay RSRz and the series resistance R2. The relays RQOI, RSR1and RQOZ, RSRz are controlled-"from their respective selector relays SM1and SMz as will be explained later in con.- nection with the descriptionof operation of the whole circuit.

The receiving control circuit ll also includes another portion, whichconsists of the two threeeleotrode vacuum tube detectors 3G and ti thecathode-anode circuits of which are differentially connected to theamplitude comparor polarised relay ACP through the two opposing windings32 and 33 thereof. The grid-cathode circuits of the detectors 3!! and 31are respectively connected across the condenser 24 in operating channel2i), and across condenser 28 in operating channel I9, so that if thecharge on condenser 25 in receiving channel 20 is larger than the chargeon condenser 28 in the receiving channel it, the relay ACP operates inthe upward direction, and if the charge on condenser 23 is larger thanthe charge on condenser 24, the relay ACP operates in the downwarddirection.

The loss network 2 in the path EA may be of any suitable type which willnormally provide a low loss to transmitted waves and which is adapted tobe changed to the high loss condition by operation of a mechanicalrelay. The particular type of network illustrated is disclosed andclaimed in H. 0. Silent Patent 1,749,851 issued March 11, 1930. Asindicated, it comprises the two transformers 34 and 35 connected intandem in the circuit EA. The transformer ("34 has one primary windingand two secondary windings 33 and 31, and the transformer 35 has twoprimary windings 38 and 39 and one secondary winding. The upperterminals of the windings 35 and 38 are connected together and likewisethe lower terminals of the windings 3'! and 39. The lower terminal ofthe winding 36 is connected to the upper terminal of the winding 39, andthe lower terminal of the winding 38 is connected to the upper terminalof the winding 3?. These connections, taken by themselves, are such thatthe windings 36, 38, 31, 39 would oppose each other in such manner as toprevent transmission over the path EA through the transformer network 2.However, as indicated a normal cross-connection made through thenormally closed armature and contact of the suppressor relay SR tiestogether the lower terminals of the windings 38 and 3t? and the upperterminals of the windings 3i and 39, so that, in effect, two seriesaiding circuits are established through transformers 34 and andtransmission through the network 2 over the path EA is normallypermitted. When the suppressor relay SR operates to open its armatureand contact, the normal cross-connection is removed and the windings 36,38, M, 39 are con nected in opposing relation so that transmissionthrough the transformer network 2 is substantially prevented.

Echo suppression apparatus identical with that just described isassociated with the output of the path EA and the input of the path WAat the east terminal (not shown) of the four-wire circuit, thetransmitting control circuit corresponding to the transmitting controlcircuit 5 at the west terminal being connected to the input of the pathWA and a receiving control circuit at the west terminal being connectedto the output of the transmission path EA. The operation of the elementsof the circuits just described as Well as that of other elementstherein, not previously referred to, in controlling transmission overthe four-wire circuit will be clear from the following description ofoperation of the entire system.

Operation In the unoperated condition of the circuit, that is with nospeech waves being transmitted in either direction and with only theusual line noise in the transmission paths EA and WA, the sensitivitiesof the transmission control circuits 5 and I? are arranged so that thesuppressor relay SR is unoperated and its armature and contact areclosed, as indicated in the drawing, and thus the loss network 2 in thepath EA is in the low loss condition. The transmission control circuitsat the east terminal of the four-wire circuit (not shown) will be insimilar condition, so that the paths EA and WA will both be operative totransmit.

In the operation of the echo suppressor circuits of the invention thereare three distinct cases to be considered: (1) operation on transmittedspeech only; (2) operation on received speech only; and (3) operation onboth transmitted and received speech.

Transmitted speech only Speech waves fortransmission in the directionfrom west to east arriving over the telephone line TL will be impressedby the hybrid coil H1 on the input of the path EA. The main portion ofthese waves will be impressed upon the input of the amplifier I and theamplified waves in the output thereof, the loss pad 2 being in the lowloss condition, will be transmitted therethrough and over the outgoingportion of the path EA to the east terminal of the four-wire circuitfrom which it will be transmitted to an east subscriber.

Part of the transmitted speech waves will pass through the directionalnetwork 4 into the input of the transmitting control circuit 5. TheseWaves will be amplified by amplifier 6, and the amplified waves will bedivided between the input of the circuit I and the input of the detectorI. The initial portion of the Waves transmitted to the input of thedetector 1 through transformer 8 will be rectified by rectifier I3 andthe rectified waves will produce an RI drop in the resistance Iconnected across the input of the detector tube 1;

The portion of the amplified speech waves diverted into the circuit IDwill be detected by the detector tube II .and the detected waves in theoutput thereof will cause operation of the relay I2 to open its normallyclosed armature and contact thereby effectively disconnecting theresistance I5 from in shunt of the control condenser I 4 in the mainportion of controlcircuit 5. This will allow the rectified speech in theoutput of rectifier I 3 to charge up the control condenser I 4 to avoltage corresponding to the peak syllabic amplitudes, which charge willremain on the condenser I4 until the relay I2 releases ,to reconnect theresistance I5 in, shunt with the; control condenser I4 when the chargewill leak off through this resistance. 7

No appreciable hang-over is applied to the control circuit I 0 so thatthe control relay I2 will be amplified by the amplifier 3.

will release, discharging control condenser I4, as soon as the speechwave has passed. The charge on the control condenser I4, as long as itremains, is applied to the input of the detector tube 1 and causes theoperation of the tube to apply energizing current to the winding 9 ofthe suppressor relay SR in such direction as to tend tohold the relayarmature more firmly on its contact thus retaining the loss network 2 inthe path EA in the low loss condition, and biasing the relay SR againstoperation by the receiving control circuit I! in proportion to theamount of current in the winding 9.

The transmitting control relay I2 is made slow operating so that theresistance I5 is not immediately removed from in shunt of the condenser.I4 in response to impressed wave energy. This prevents the transmittingcontrol condenser I4 from receiving a charge from an echo in the path EAof the first syllable of speech energy to be received over the path WAand thus permits the receiving control circuit I! to operate thesuppressor relay SR quickly in response to received speech. Fastoperation of the suppressor relay SR is necessary in order to minimizeinitial echoes.

Received speech only The operation of the suppressor apparatus at theeast terminal of the four-wire circuit in response to the speech wavestransmitted from the west terminal over the path EA will be clear fromthe following description of operation of the suppressor apparatus atthe west terminal in response to speech waves received from the eastterminal over the path WA.

The speech waves received over the path WA The main portion of theamplified waves will be transmitted out over the outgoing portion of thepath WA and will be impressed by the hybrid coil H1 upon the two-waytelephone line TL over which it will h e-transmitted to the westsubscriber.

A portion of the speech waves transmitted through the hybrid coil H1 tothe two-way line TL will return and be transmitted into the input of thepath EA in the form of an echo. The initial echo, due to the first partof the received speech syllables will build up a voltage acrosscondenser I4 which will be small due to the parallel resistance I5. Thusit will not be effective to apply a full bias against the operation ofthe receiving side of the suppressor before the suppressor relay SR hasbeen operated by the receiving side, due to the slow operation oftransmitting control relay I2, as described above.

Meanwhile, the portion of the amplified received speech waves divertedinto the receiving control circuit I! will be amplified by theamplifying device I8 therein.

First, let it be assumed that the receiving control circuit is of theusual type with only one channel. In that case, in actual conversationthe receivedspee ch syllables would follow each other so closelythat thereceiving control circuit relay with the usual hang-over (about 0.1seconds) would very frequently not fall back between syllables. As aresult, a weak syllable following a strong one which had causedoperation of the suppressor relay SR, might maintain the voltage on thecontrol condenser produced by the strong syllable. The current in thewinding 26 oi the suppressor SR, which acts as a bias against break-ins,will then tend to be unnecessarily high, and since in a normalconversation at trolling the supp'ressorrelay, the number of chanto theupper contact.

nels being such that each particularchannel will have been returned to'normal before the next operating syllable comes along. The number of.

channels required depends on the rate at which signals are beingtransmitted and the hang-over period of the holding currents. For theusual speech signals, two channels, as illustrated, will be sufficientto produce satisfactory operation.

The operation of the receiving channels will now be described.

A portion of the amplified speech waves in the output of amplifier [8will be divided between the input of the receiving channels l9 and 20.Another portion will be diverted into the selector circuit 2|. It willbe assumed that when the first syllable is received, the receivingcontrol 1 circuits are in the condition indicated in the drawing withthe armature of the relay SP on its right-hand contact so that thewinding of the relay 8M2 is short-circuited, and with the armature ofrelay ACP on the lower contact so that the output of detector 29 isconnected to the winding 26 of relay SR and the output of detector isopen.

The first syllable of the received speech waves in the output ofamplifier I8 will be diverted into control circuit 2| and will bedetected by the detector 22 therein. The detected current will passthrough the winding of the relay SlVll causing operation of that relayto shift its armature from the right-hand contact to the left-handcontact.

When this happens, a voltage is applied from battery it to the windingsof the relays RSR1 and RQOI in series through the left-hand contact andarmature of relay SM1, thereby causing operation of the former relays tobreak their armatures and contacts. This will remove theshort-circuiting connections across the receiving channel 2% and allowthe speech impulse diverted into that channel to charge condenser 24through rectifier 23 to the maximum voltage of the rectified wave. Thisrectified voltage is supplied to the grid of the detector tube and theresulting detected current passing through the winding 32 of relay ACPwill cause operation of that relayto shift its armature from the lowerThis disconnects the output of rectifier 29 in receiving channel it fromthe winding 26 of the suppressor relay SR, and connects the output ofthe detector 25 in receiving channel 20 thereto. Thiswill allow theoutput current of the detector 25 to build up in the winding 25 of therelay SR sufficiently to cause operation of that relay to break itsarmature and contact, thereby changing the loss network 2 in the path EAfrom the low loss to the high loss condition and efiectively suppressingechoes of the received speech current in that path.

The charge produced'on condenser i l in the transmitting control circuitby the echo of the received speech current in the path EA, willproduceabias on the opposing winding 9 of the suppressor relay SR, inthe manner previously described but this bias will not be sufficient tocause that winding to prevail over the winding 26 conference inamplitude between the received speech and its echo. Relay SR thereforewill be maintained operated to suppress the echo in the path EA.

When the relay SM1 operates in the manner which has just been described,in response to a received speech syllable, the condenser ll associatedwith relay SP will be charged by current from the battery 50 through theleft-hand contact and armature of relay SMl, but the charging current isso directed through the winding 42 of relay SP that the armature thereofis held more firmly on its right-hand contact. At the end of thesyllable (i. e. when the syllable amplitude has fallen below the operatepoint of the selector detector 22) the relay SM1 will releasepractically immediately. The condenser M will then be discharged throughthe winding 42 of the relay SP causing the relay to shift its armaturefrom the right-hand to the left-hand contact, thereby shorting thewinding of relay SM1 and removing the short from the winding of therelay SMz.

Also, at the end of the speech syllable, with the release of SM1, therelay RQO1 will immediatelyrelease reconnecting the normal short circuitacross that channel through the contact and armature of that relay. Therelay RSR1, however, due to its slow-releasing characteristic, will stayoperated for a short hang-over interval of time so that the suppressorrelay SR will maintain the high loss in the path EA until the last partof the echo has been dissipated.

The portion of the second received speech syllable diverted into theselector circuit 2! will be detected by the detector 22 therein and willcause operation of the relay SMz to shift its armature from theright-hand to the left-hand contact. This will cause energizing currentfrom battery 33 to be supplied in series to the windings of the relaysRSRz and RQOz associated with the receiving channel l9, causing theiroperation to open the normal short-circuits across that channel in theinput and the output of rectifier 2?. This will allow the other portionof the second receive-d speech syllable in the output of amplifier l8 tostart to build up a voltage across condenser 28 in receiving channel l9through rectifier 21.

If the second speech impulse is stronger than the preceding speechimpulse the voltage built up on condenser 28 will exceed the voltagepreviously built up by the latter impulse on condenser 24 in receivingchannel 20. In that case, the rectified voltage on. condenser 28 actingthrough the detector tube 3! of the differential detector will produce acurrent of such magnitude in the winding 33 of the amplitude comparorrelay ACP as to cause the armature of that relay to be thrown to itslower contact thereby disconnecting the output of detector 25 in thereceiving channel 29 from the receiving winding 26 of the suppressorrelay SR and connecting the output of the detector 29 in the receivingchannel l9 thereto. The latter detector will transmit current to thewinding 26 of the suppressor relay SR in such direction as to tend tohold that relay operated to maintain the network 2 in the path EA in thehigh loss condition. The current in winding 26 will build up to a levelwhich is proportional to the strength of the second received speechimpulse, which will be just sufilcient to prevent the release of relaySR on the echo of the second speech syllable in EA operating through thetransmitting control circuit 5.

If the second received speech impulse were 7 weaker than the first sothat the voltage on condenser 28 in receiving channel l9 does not exceedthe voltage on the condenser 24 in receiving channel 2d, the armature ofthe relay ACP would be maintained on its upper contact and the currentin the winding 26 of the suppressor relay SR will be maintained at amagnitude corresponding to the charge on condenser 24. Thus, it will beseen that the bias on relay SR through the receiving winding 26 of thatrelay will always be held at a value proportional to the stronger ofeach two succeeding received speech syllables for time intervals justsufficient to prevent false operation of the relay SR by the echo of thestronger syllable to change the network 2 in the path EA from the highloss to the low loss condition.

At the end of the second syllable, relay SMZ will release practicallyimmediately and the con denser M, which was charged by current frombattery d3 during the period in which the relay SMa was maintainedoperated, will discharge through the winding 45 of the SP relay causingthe armature of the latter relay to return to its right-hand contact,thereby short-circuiting the winding of the relay SM, .and removing theshort from the relay SM1. Also, with the release of relay SMz, the relayRQOz associated with the receiving channel it! will release reblockingthe input of channel it by the resulting short-circuit through thearmature and contact of the latter relay. The relay RSRz will remainoperated for a small interval of time after relay SMz has released, dueto its slow releasing characteristic, so that the relay SR will bemaintained operated to maintain the path EA blocked until the echo hasreturned to the suppressor point.

If a weak syllable follows the preceding strong syllable, the relaySMlwill be operatedby it in the manner previously described, followed byoperation of the relays. RQOI and RSRi associated with the receivingchannel 26. The voltage on condenser 24 will then build up, but not toas high a value as that on the condenser 28 associated with thereceiving channel [9. Therefore, the armature of the relay ACP. willstay on its lower contact until, and only as long as, the

relay RSRZ is held operated by its hangover, after which the armature ofthe ACP relay will be thrown to the upper contact causing the charge onthe condenser 24 to control the winding 26 of the suppressor relay SR.

Operation on both transmitted and received speech When speech Waves fromthe subscriber associated with the west terminal of the four-wirecircuit and the subscriber associated with the east terminal of thefour-wire circuit are present simultaneously in the input of the path EAand the output of the path WA, respectively,'there are three possibleconditions of operation of the suppressor apparatus: (A) speech arrivingat the receiving side first; (B) speech arriving atthe transmitting sidefirst; and (C) speech arriving at both sides simultaneously.

In condition (A), considering the west terminal of the four-wirecircuit, the successive syllables of the received speech waves from thepath WA, in the receiving control circuit l'i alternately produceproportional charges on the condenser 24 in the receiving channel 20 andcondenser 28 in the receiving channel l9, which, in the mannerpreviously described, produces a current in the winding 25 of thesuppressor relay SR proportional to the stronger of the syllables, andcausing the operation of the relay SR to condition the loss network 2 inthe path EA to provide a large loss therein in the manner which has beenpreviously explained in detail. The portion of the outgoing speech waveslater received by the transmitting control circuit 5 subsequentlyproduces a charge on the control condenser M therein in the manner whichhas been previously described. Each charge is proportional to the peakpower of the input waves.

If the charge on the transmitting control circuit condenser M is greaterthan that on the receiving circuit condenser 24 or 28 due to the westsubscriber talking more strongly than the east subscriber, the winding 9of the suppressor relay SR will prevail over the winding 26 oithat relaycausing the relay to release and thereby condi tions the loss network 2in the transmitting path TA to permit the transmitted speech to pass.But, if the charge on the receiving condenser 24 or 28 is sufiicientlygreater than the charge on the transmitting condenser M, the suppressorrelay SR will not be released, the path EA will remain suppressed andthe received speech will retain control of the suppressor circuits.

In condition (B) the portion of the transmitted speech diverted into thetransmitting control circuit 5 from the path EAwill produce a charge onthe transmitting control condenser it. This will bias the detector i sothat a proportional current will be produced in the winding 9 of thesuppressor relay SR in its output which will be in such direction as totend to hold the relay armature more firmly on its contact, whereby theloss network 2 in the path EA-will be maintained in the normallow losscondition. Speech arriving later on the receiving side and diverted fromthe path, WA into the receiving controlcircuit I! will place aproportional charge on the receiving control condenser 24 or 28, aspreviously described. The latter charge will not cause the suppressorrelay SR to operate to change the loss network 2 in the path EA to thelarge loss condition unless this charge: is. sufficiently greater thanthat on, the transmitting condenser M. v

In condition (C) charges: are placed on both the transmitting andreceiving control circuit condensers by the west subscribers and theeast subscribers speech currents, respectively; at the same time. If thecharge on the receiving circuit condenser is sufficiently greater thanthat on the transmitting circuit condensers, the receiving side controlsthe suppressor relay, otherwise the transmitting side controls thesuppressor relay.

It will be apparent from the above description of operation of thereceiving control circuit, that by using the two similar receivingchannels operating on successive syllables for insuring that the amountof current in the receiving winding of the suppressor relay isproportional for the required time to the stronger of the receivedsuccessive signals, the bias against false operation of the suppressorrelay by echoes will only be just sufiicient to prevent false operationby the echoes of these stronger signals. Because the bias produced bythe receiving side on the suppressor relay at any time is only just theminimum amount necessary, break-ins by either party is facilitated,While at the same time echoes are satisfactorily suppressed.

It is to be understood that the invention is not limited to the precisedetails of the circuits illustrated and described, for it is apparentthat numerous modifications thereof could be made by those skilled inthe art within the spirit and scope of the invention.

What is claimed is:

1. In a two-way signal wave transmission sys tem comprising at eachterminal a one-way signal transmitting path and a one-way signalreceiving path, a difierential type echo suppressor assoclated with thesignal transmitting and signal receiving paths at each terminal of thesystem for directionally controlling signal transmission therein, andmeans for insuring that the bias against break-ins applied to thetransmitting side of each echo suppressor by the receiving side thereofis always proportional to the amplitude level of the stronger ofsuccessive signal impulses in the signal receiving path.

2. In a two-Way signal wave transmission system including a four-wiresignal repeating circuit, an echo suppressor of the differential typeassociated with the outgoing and incoming two-wire paths at eachterminal of said four-wire circuit for directionally controlling signaltransmission over said four-wire circuit, the one portion of the echosuppressor controlled from the incoming two-wire path operating to biasthe other portion controlled from the outgoing two-wire path againstoperation, said one portion of said echo suppressor including means forcomparing the amplitude level of successive signal impulses receivedfrom the incoming two-wire path, and for utilizing the stronger of thecompared impulses to exclusively produce a bias on said other portion ofsaid echo suppressor, proportional to its amplitude until itstermination.

3. In a two-way telephone transmission system comprising at least nearthe terminals thereof oppositely directed one-way transmission paths forthe speech wave signals transmitted in opposite directions, an echosuppressor unit at a terminal of the system comprising wave-controlleddifferential means which when operated in one direction disables one ofthe one-way paths near that terminal and when operated in the oppositedirection tends to prevent said one one-way path from being disabled,control circuits respectively connected to said one one-way path and theother one-way path near said terminal, for transmitting controlling waveenergy therefrom to said difierential means, said control circuits beingresponsive to stronger speech signal Waves received from said otherone-way path than from said one one-way path to cause operation of saiddifferential means in said one direction and responsive to strongerspeed signal wave-s received from said one one-way path than from saidother one-way path to cause operation of said differential means in saidopposite direction, and means in the control circuit connected to saidother one-way path for insuring that the amount of controlling waveenergy transmitted thereby to said differential means is proportionalfor a definite period to the amplitude level of the stronger ofsuccessive syllables of the speech signal Waves'received from said otherone-way path.

4. In a two-way telephone transmission system comprising at least nearthe terminals there- 'of oppositely directed one-way transmission pathsfor the speech signal waves transmitted in opposite directions, an echosuppressor unit at each terminal of the system comprising wavecontrolleddifierential means. which when operated in one direction disables theoutgoing one- Way path at the terminal, and Wh p a ed. i

the oppositedirection tends to prevent said outgoing one-way path frombeing disabled, a transmitting and a receiving control circuitrespectively connected to the outgoing one-way path and the incomingone-way path at the terminal, for transmitting controlling wave energytherefrom to said differential means, the control circuits be ingresponsive to stronger speech signal waves received from said incomingone-way path than from said outgoing path to cause the operation of saiddifferential means in said one direction and responsive to strongerspeech signal waves received from said outgoing path than from saidincoming path to cause operation. of said differential means in saidopposite direction, and means for facilitating break-ins by a listeningparty when the speech signal Waves of one talking party have obtainedcontrol Otf said echo suppressor unit comprising means in said receivingcontrol circuit for comparing the amplitude levels of successivesyllables: of the speech wave energy received thereby, and for utilizingthe stronger of the compared syllables to exclusively control theoperation of said difierential means in said one direction.

5. The system of claim 4, in which said means in said receiving controlcircuit comprises a plurality of receiving channels connected inparallel to the incoming one-way path, means for directing successivesyllables of the received speech signal waves alternatively into saidchannels, means for comparing the amplitude levels of the syllables inthe several channels, and means for selecting the strongest of thesyllables in the several channels, and utilizing it to control thedifferential means in proportion to its amplitude level.

6. The system of claim 4, in which said differential means includes adifferential relay having two opposing windings energized from thetransmitting and receiving control circuits, respectively, and saidmeans in said receiving control circuit comprises a plurality ofreceiving channels connected to said incoming path, means for directingsuccessive impulses of the speech waves received from said incoming pathinto saidreceiving channels alternately, means continuously comparingthe amplitude levels of the impulses in the several channels and meansresponsive to the amplitude level of the strongest impulse in theseveral channels to supply a corresponding amount of energizing currentto the receiving circuit winding of said diflerential relay.

'7. The system of claim 4 in which said differential means includes adifierential relay having two opposing windings respectively connectedto the transmitting and the receiving control circuit, and said means insaid receiving control circuit comprises a plurality of receivingcontrol channels, means for directing successive syllables of the speechsignal waves received from said incoming path alternately into saidchannels, means responsive to the first speech syllable received throughthe channel in which directed, to cause energizing current ofproportional amplitude to be supplied to the receiving circuit windingof said differential relay, means to compare the amplitude of said firstsyllable with the amplitude of the second received syllable in anotherchannel, means responsive to the second speech syllable, only ifstronger than the first syllable, to cause energizing current ofproportional amplitude to be supplied to said receiving circuit windingin place of the energizing current due to the first syllable, and meansto repeat these steps for the succeeding syllables of the signal wavesin the other channels.

8. The system of claim 4 in which said differential means includes amechanical relay with two opposing windings respectively connected tothe output of the transmitting and receiving control circuits, and saidmeans in said receiving control circuit comprises a plurality of controlchannels, means to direct the first syllable of the received speechsignals into one of said control channels, means responsive to theenergy of said first syllable in said one channel to supply energizingcurrent of proportional amplitude to the receiving winding of saidrelay, means to direct the next syllable of the received speechsyllables into another of said control channels, means responsive to agreater amplitude level of said next syllable in said other channel thanthe level of said one syllable diverted into said 7 one channel, tocause the energizing current supplied to the receiving winding of saidrelay to become proportional in amplitude to the arm plitude level ofsaid next syllable, and responsive to: a greater amplitude level of thesyllable in said one channel than that of said next syll able in saidother channel to maintain the enerand said means in said receivingcontrol circuit comprises a plurality of parallel control channels eachincluding a vacuum tube detector, the output of one of said detectorsbeing connected through the contacts of a second relay to the receivingcircuit winding of said differential relay, and the outputs of the otherdetectors being adapted to be connected thereto in place of the outputof the first detector through other contacts of said second relay, meansfor diverting the first syllable of the received speech signals fromsaid incoming path into one of said control channels, means responsiveto the wave energy of said first syllable in said one channel forcontrolling said second relay to connect said one control channel, ormaintain it connected if already connected, to the receiving circuitwinding of said diiferential relay so that the detected current in saidone control channel will control said differential relay in accordancewith its amplitude, means for diverting subsequent signals of thereceived speech signals respectively into other control channels, andmeans responsive to the Wave energy in one of said other channels ifstronger than that in said one channel to control said second relay toconnect the output of said one other channel to the receiving winding ofsaid differential relay in place of the output of said one channel sothat the differential relay will be controlled by the amplitude of thestrongest received speech syllable.

BJORN G. BJORNSON.

